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PULP&PAPER Mechanical design means of things and systems of a mechanical nature-machines, products, structures, devices, and instruments. For the most part mechanical design utilizes mathematics, the materials sciences, and the engineering-mechanics sciences.43507
The actual practice of designing is applying a combination of scientific principles and a knowing judgment based on experience. It is seldom that a design problem has only one right answer, a situation that is seemingly best one being chosen.
Early in the 20th Century, conical refining evolved away from beaters used for fiber defibrillation (i.e., why some mills still call the refining area "the beater room"). The conical fit the requirements of the time quite well, because it was very good for shortening the long, non-wood fibers then in use. The conical continued to be applicable for stock refining into the 1960s because, as wood fibers began to replace non-wood fibers, long fiber softwoods were used, and cutting was still desired.
However, the tackle system of the older conical refiners limited their ability to fibrillate without cutting. This became more and more of a drawback as short-fiber hardwoods entered the stock furnish. The shorter hardwood fibers require fibrillation with minimal or no cutting to achieve their best papermaking potential. Refining intensities down to 0.2 Ws/M are recommended, 1 and this is usually not possible with the older style conicals.
Conical refiners also had many operational problems. They required significantly higher power, making them energy inefficient. The fillings take 8 to 24 hrs to change, are expensive, and have long lead times. It is often necessary to have 30% more installed refining capacity than is demanded by the furnish requirements to compensate for refiners being down for filling changes.
More uniform fiber treatment by the conical refiner is an apparent explanation for its superior results. Because the speed of the moving bars in a refiner determines the quality of treatment of the fibers and the amount of energy required to shear the pulp slurry, an ideal refiner would have all positions along the bars moving at equal speeds. This suggests a cylindrical refiner, but a cylindrical refiner which allows a pulp slurry to be passed through it continuously in an axial direction has been found to be almost impossible to design feasibly. The closest and most successful approach to this ideal configuration is the conical refiner.
The disc refiner evolved to replace the conical refiner. In general, disc refiners are able to operate at a higher RPM and use refining plates with a longer cutting edge per unit area. This equates to a lower refining intensity suitable for hardwoods. Disc refiners require relatively less power than the conicals, equaling a lower cost per ton for operation. The cast plates are easier to change, are easier to obtain, and offer more versatility in bar patterns.
But disc refiners also have their problems. At the larger diameters required for high- speed paper machines, their operating RPM is limited by the circumferential speed at the outside diameter. This limits the ultimate ability to achieve the ultra low refining intensity beneficial to hardwood fibers and mechanical pulps. Due to the vortex flows, stock flow, and centrifugal forces, not all the fibers presented are refined, since some follow the plate grooves from inlet to the discharge.2 It has been surmised that, in some cases, as little as 30% of the fibers get refined in the first pass through a disc refiner. In these instances, refining efficiency and energy efficiency is low.
In a study inserting plastic tracer fibers in a refiner stock flow, the Pulp & Paper Centre of the Univ. of British Columbia determined that many fibers showed no evidence of impact. And the fibers that did were severely deformed, indicating an over-refined state.3,4
Therefore, in a disc refiner, it is probable that the fibers that are impacted tend to be over-refined to compensate for those that are not impacted to achieve the overall desired freeness drop. This leads to undue generation of fines, weakening of the refined fibers, and inefficient delivery of energy to the fiber. In another study, Martinez and Kerekes showed that in a single-bar lab refiner, only 0.1% of the refining energy expended resulted in tensile strain of a fiber.4,5